Those who have much to do with chilled irons will find the etching test a valuable one. While the practised eye alone can arrive at the true valuation of what the etched surface shows, yet the test is so simple that the operation should be understood generally. The greatest development has naturally been in the line of the steels. First, to distinguish between these and wrought iron and thus readily detect fraud and substitution. Second and later, to get at the actual crystalline structure in order to judge the quality as affected by the heat and mechanical treatment the specimens had received.
For cast iron, the polished and etched surface shows up the nature of the crystalline structure in the chilled portion, and the gradation into gray iron. Where experiments are made with additions of steel or wrought scrap, the appearance of the etchings is a guide to the probable wearing qualities. The samples must be first prepared by filing or grinding to get a flat surface. Then this is.smoothed with successive grades of emery cloth until a bright surface is obtained which is not too deeply scratched. This polished surface must not be touched with the fingers, as anything of a greasy nature prevents the acid from attacking the iron. Now the piece is immersed face up in nitric acid diluted with ten parts of water. It is best to use this mixture cold. A few seconds will suffice to bring out the structure. The test piece is then taken out and washed thoroughly in running water.
* This article on etching was contributed to this work by the kindness of Dr. Richard Moldenke.It is also being introduced to the trade in England from H. Bertram & Co., 28 Queen street, London, E. C., who offer to supply full particulars.'' il